Crime/disaster prevention systems (e.g. alarm systems) are generally made from wireless communication systems because the construction and installation of wireless systems is relatively simple. For example, many alarm systems comprise a sensor and a receiving unit. The sensor may be located near a window or an entrance to detect the presence of an intruder or fire. The receiving unit may monitor the status of the sensor and may transmit information to a remote control center if the sensor detects an intruder or fire. Moreover, in order to facilitate the installation of the receiving unit and the sensor, such devices may not be connected via wires but may communicate via the transmission and reception of radio waves.
However, several restrictions are placed on the operation of such a wireless alarm system. In particular, many of the limitations arise due to the legal restrictions which prevent the use of radio waves having particular frequency and output ranges. Accordingly, the many alarm systems cannot communicate via the types of radio waves which are best suited for communicating indoors.
For instance, some alarm systems are forced to communicate via weak radio waves having frequencies of several hundred megahertz. In an open area, such weak radio waves enable smooth and clear communication between a receiver and a transmitter which are located several tens of meters apart from each other. However, when the receiver and transmitter are located within an indoor room, the weak radio wave collides with and reflects off of various structures within the room (e.g. walls, ceiling, floor, furniture, etc.). Consequently, the radio wave dissipates as it is travelling to the receiver as a result of such reflections and collisions, and the intensity of the wave sharply decreases. The phenomenon above is typically called fading.
A radio wave having a frequency of several hundred megahertz has a wavelength on the order of one meter. Therefore, since the dimensions of indoor rooms are on the order of several meters, fading occurs at many places inside indoor rooms. If the orientation of the objects within an indoor room changes (e.g. furniture is rearranged or people walk through the room), the locations in the room where the fading of a radio wave occurs likewise varies. Furthermore, the communication between a sensor and a reception unit may often fail when the orientation of objects within the room only changes slightly, even though communication could be maintained up until such slight change.
In addition, the communication between a sensor and a reception unit is also adversely effected by noise produced from electric appliances such as televisions and refrigerators. Thus, the distance over which the sensor and reception unit can properly communicate becomes very short, and often, the sensor cannot appropriately inform the reception unit that it has detected an intruder or a fire. Accordingly, the alarm system may have to be redesigned so that the sensor is relocated in the room such that it can adequately communicate with the reception unit. Alternatively, a wire may have to be used to provide a communication path between the sensor and the reception unit, but such solution contradicts the entire purpose of a wireless system.
In order to ensure that the alarm system is operating properly, the reception unit must periodically communicate with the sensor to determine the status of the sensor and confirm that it is functioning correctly. Consequently, the sensor may supply relevant information to the reception unit in one of two manners. First, the reception unit and the sensor may be designed to communicate in a bidirectional mode. In such mode, each of the reception unit and the sensor comprises a transmitter and receiver. Therefore, when the reception unit needs information, it transmits an information request signal to the sensor, and the sensor transmits the requested information to the reception unit in response to such signal. Also, the two devices may be designed to communicate in a fixed-time alarming mode. In this mode, the sensor comprises a transmitter, and reception unit comprises a receiver. Thus, the sensor periodically outputs a signal corresponding to its status without the reception unit previously requesting such information.
An alarm system which has a reception unit and a sensor which communicate via the bidirectional mode is very reliable and versatile. For instance, if a person who is in the room is authorized to be in the room, the reception unit can be programmed in accordance with such fact. Thus, the reception unit can send a command to disable the sensor so that the sensor does not erroneously determine that the authorized person is an intruder. Furthermore, if the sensor is disabled, the reception unit can still transmit a command to the sensor to determine if the sensor is still operating properly. In response to such command, the sensor transmits a status command back to the reception unit.
However, a system which uses the bidirectional communication mode has some disadvantages. For example, since an alarm system typically contains a plurality of sensors, installing a receiver in each sensor is relatively expensive. Furthermore, in order to receive a radio signal having a wavelength of about one meter, each sensor must comprise an antenna having a length of approximately 20 to 40 cm, and thus, the size of the sensor is significantly increased. Using a large sensor in an alarm system defeats the purpose of the alarm system because the sensors ideally should be concealed from potential intruders. In addition, using a large antenna for each sensor detracts from the aesthetics of the room.
The fixed-time alarming mode is advantageous because the sensor only contains a transmitter, and the reception unit only contains a receiver. Moreover, the transmitter located in the sensor is simpler than the receiver located in the sensor described above. Therefore, the antenna needed to transmit radio waves from the sensor can be a print pattern which is only several centimeters long. In addition, the power consumed by a transmitter is much smaller than the power consumed by a receiver. Accordingly, the sensor can easily operate by using a battery as a power supply. As a result of the advantages above, alarm systems using the fixed-time alarming mode are more widely distributed than systems using the bidirectional communication mode.
However, an alarm system using the fixed-time alarming mode has several disadvantages. For example, the sensor typically transmits its status to the reception unit less than once every ten minutes. Moreover, since communicating via weak radio signals is not extremely reliable, the reception unit waits until it has not received a status signal from the sensor on multiple instances before determining that the sensor has malfunctioned. However, such procedure lengthens the time needed for the reception unit to detect that the sensor has malfunctioned.
In addition, the reception unit is typically connected to a remote control center and has a dialing device which automatically calls the control center if the sensor has detected a problem. However, if a fire or intruder is present in the room and disables the sensor before the sensor can detect such presence, the reception unit waits to receive a status signal from the sensor at multiple instances before calling the control center to inform the center that the sensor has malfunctioned (or has been disabled). Since the sensor may transmit a status signal to the reception unit only once every ten minutes, the reception unit may wait 30 or 40 minutes (i.e. attempting to receive the status signal three or four times) before contacting the control center. As a result, the control center is unable to contact the proper authorities to quickly respond to the fire or intrusion. (Furthermore, since an alarm system using the bidirectional communication mode may also wait before contacting the control center, such system also suffers from the disadvantage above).
In order to overcome the disadvantages of wireless alarm systems which communicate via radio waves, an alarm system comprising a sensor which communicates in the fixed-alarm mode by transmitting ultrawaves having a frequency of 25 KHz has been proposed. In such system, the sensor comprises a transmitter having a speaker that is made of a ceramic device 10 mm in diameter. Alternatively, the reception unit may comprise a receiver made of such ceramic device.
In addition, the alarm system employs a simple modulation process in which a "1" and a "0" are represented by the presence or absence of the ultrawave transmission. Therefore, the configuration of such system is simplified, and its cost is reduced. Furthermore, ultrawave transmissions are much less effected by interference than radio wave transmissions.
However, the communication speed of an alarm system using ultrawaves is very slow. Specifically, in order to avoid a data error caused by interference between two sequential signals, a new signal cannot be transmitted until the reverberation of the ultrawave corresponding to the preceding signal disappears. Consequently, the above alarm system takes several minutes to confirm the status of all of the sensors of the system, and thus, the system may not be able to quickly respond to an emergency. Furthermore, if an object is placed in the communication path between the sensor and the reception unit, a communication error will occur because the ultrawaves will be sharply attenuated. In light of the disadvantages above, few alarm systems communicate via ultrawaves.
Also, the alarm systems above use batteries for power supplies in order to reduce the amount of wiring needed to install the systems. Accordingly, such systems use various techniques to attempt to extend the life of the batteries. For example, battery life may be extended by controlling the duty cycle or by lengthening or shortening the period during which the battery supplies power to various parts of the system. However, if such period is lengthened (e.g. lengthened to about one second), the communication time between the sensor and reception unit is likewise lengthened. On the other hand, if such period is shortened (e.g. shortened to about several milliseconds), an expensive high precision transceiver is required because the operation of the system must be able to stabilize quickly.